Touch screens are electronic visual displays that can detect the presence and location of a touch on the surface of the display area. Touching of the display is generally done with a finger or hand. Touch screens operate under a variety of electronic, acoustic or optical principals. This application is concerned with capacitive touch screens.
Capacitive touch screen panels generally include an insulator, such as glass, coated with a transparent conductor, such as copper or indium tin oxide (ITO). Because the human body is also a conductor, touching the touch screen results in a measurable change in capacitance. The change in capacitance caused by the touch is localized and registered to a particular location on the touch screen.
Capacitive touch sensing technologies including discrete touch pads and multi-touch screens have recently gained great acceptance in products ranging from cell phones to large display monitors. Many believe the success of these technologies is a direct result of the improved user interaction as experienced by the users.
One benefit of using a solid state touch sensing technology is its virtually unlimited life. Unlike mechanical alternatives which have moving components that wear with time in repeated use, a solid state touch sensing screen has no such limitation. Solid state touch sensing screens rarely fail and users worry little about a broken user interface. Capacitive touch sensors can be integrated underneath a single solid sealed surface, such as glass or molded plastic, which makes the sensitive components inside the product separated from and largely immune from the outside environment. This is very difficult and costly to achieve with mechanical alternatives. Thus, capacitive touch screen technologies provide great benefits for products that are used in harsh outdoor environments, industrial facilities and other locations that are subject to dirt and moisture.
In a typical implementation of a capacitive touch sensing device, the target touch sensing pad, which is typically a square, rectangular or circular area of copper or indium tin oxide (ITO) on a carrier such as glass reinforced epoxy laminate (FR4), printed circuit board (PCB) or polyethylene terephthalate. (PET). The target touch sensing pad is actively charged then permitted to passively discharge at a rate which is proportional to its natural capacitance. The rate of discharge of the target touch sensing pad is measured using one of several well known methods. When a finger or other conductive appendage is placed over the touch sensing pad, the presence of the finger increases the capacitance of that pad by adding to the pad's natural capacitance. In this state, the touch sensing pad is able to hold more charge and as a result takes longer to discharge. By measuring the difference in the time it takes to discharge a particular touch sensing pad in the two states, one can determine if the pad is being touched or not.
The amount of increase in capacitance when a finger is placed against the touch sensing pad varies dependent upon the design and construction of the touch sensing pad. The greater the capacitive coupling between the finger and the touch sensing pad, the greater the change in capacitance. Conversely, the less the coupling between the finger and the touch sensing pad the less the change in capacitance due to the touch. Higher changes in capacitance when the touch sensing pad is touched yield a higher signal to noise ratio (SNR) which translates to better performance of the touch sensing pad. The proportion of increase in capacitance of the touch sensing pad when it is touched by a finger is a function of the natural capacitance of the pad and the added capacitance provided by the presence of the finger. Accordingly, if the pad has low natural capacitance coupled with a better coupling to a human finger, better sensitively and performance to touch will be demonstrated.
The natural capacitance of a touch sensing pad is determined by several factors. The choice of materials used in construction of the pad including but not limited to the material of the carrier (which is the dielectric substrate to which the conductive sensor is attached), the protective substrate (which is the surface behind which the sensor is protected) and the conductors. The placement of other conductors around the touch sensing pad and the electrical potential on those conductors also affects the natural capacitance of the touch sensing pad. The coupling between the conductor and the protective substrate also affects the natural capacitance of the touch sensing pad. There are many other factors and this list should not be considered to be exhaustive. Accordingly, the approach of seeking lower natural capacitance with better coupling to a human finger inherently fixes and affects the natural capacitance of the touch sensing pad. This also limits the ability to affect the SNR without completely altering the construction of the sensor. Altering the construction of the sensor is difficult and expensive and can even be impossible. The presence of these limitations tends to lead to poorly performing or very expensive solutions. Accordingly, there is room for the improvement in the area of capacitance touch sensing screens.